Flood is generally excluded from homeowners policies, so flood coverage comes mainly from the federal National Flood Insurance Program (NFIP), written on the Standard Flood Insurance Policy (SFIP).
The SFIP defines flood as a general and temporary condition of partial or complete inundation of two or more acres of normally dry land, or of two or more properties, from causes such as overflow of inland or tidal waters. A creek overflowing onto four acres of normally dry land is a flood.
Not a flood under the SFIP:
A burst water heater that fills one basement, because it is confined to one dwelling.
Rain that comes in through a roof damaged by wind.
A landslide or slope failure.
Mudflow vs. landslide: the SFIP includes mudflow in its flood definition and describes it as a river of liquid and flowing mud on normally dry land, as when earth is carried by a current of water. Landslide, slope failure and a saturated soil mass moving down a slope are not mudflows. Earth movement is excluded even if flood causes it, so water being involved does not by itself make a slide a flood.